**9. Magnetic resonance imaging assessment of plaque morphology**

Magnetic resonance imaging (MRI) is a promising modality for characterisation of carotid plaque morphology and assessment of composition of atherosclerotic plaques. It can accu‐ rately identify the presence of ulcerated or thin plaque cap, [105-107] quantify intra-plaque haemorrhage [105-107], or the presence of a large necrotic plaque core [105-107]. Serial MRI examinations in asymptomatic patients with moderate (50-70-percent) ICA stenosis have revealed correlation between these plaque findings and development of subsequent ipsilateral ocular and ischaemic neurological events [108] (Figure-5).

**Figure 6.** Automated segmentation of bright–and black-blood, high-spatial resolution, Multi-contrast in vivo MR im‐

Carotid Plaque Morphology: Plaque Instability and Correlation with Development of Ischaemic Neurological Events

http://dx.doi.org/10.5772/57254

95

High resolution MRI can identify and age intra-plaque haemorrhage. [113-115] Prospective serial MRI studies have demonstrated that haemorrhage in atherosclerotic plaques is associ‐ ated with sudden increases in plaque volume, necrotic core, and progression of degree of

In addition to Duplex and MRI, other modalities such as fludeoxyglucose (FDG) positron emission tomography (PET) CT scanning has been used to assess the level of metabolic activity in carotid atherosclerotic plaque. This is used in turn as a surrogate marker of plaque insta‐

PET CT scanning has shown some promise as a tool for assessment of plaque instability. [116] However it is unlikely to gain mainstream applicability due to its limited availability, expense, and the significant exposure to ionising radiation (meaning serial assessments are not possible) and availability of non-invasive accurate imaging modalities to assess plaque morphology.

Over the last 20 years the advances in technology have led to the evolution of non-invasive imaging modalities with high spatial resolution. The application of this technology in the assessment of carotid plaque morphology has advanced our understanding of the natural history of atherosclerotic lesions more than the assessment of histological characteristics of atherosclerotic plaques. Consequently for the first time, plaque morphology could be assessed against the two functions that ultimately matter the most: time and occurrence of future

ages compared with histological characteristics of plaque. (*JACC Cardiovasc Imaging.* 2009; 2:883-96.)

stenosis. [115]

**10. Conclusion**

embolic ischaemic events.

bility.

**Figure 5.** Identification of intra-plaque haemorrhage using high spatial resolution, multi-contrast MRI image. (*JACC Cardiovasc Imaging.* 2009; 2:883-96.)

One of the strengths of MRI is the availability of multi-contrast weighted protocols. The most common application of carotid MRI remains the acquisition of an angiogram which uses a bright blood sequence using a 3-dimensional time of flight sequence. This attenuates the signal from stationary (plaque) tissues. Black blood sequences eliminate the luminal signal and help to characterise plaque morphology. [105-109] Combining the information, multiple-contrast weightings can be used to identify all plaque components. [105-110] Plaque compositional characteristics can be assessed using automatic classifiers such as morphology enhanced probabilistic plaque segmentation (MEPPS) algorithms with a high degree of accuracy (Figure-6). [111, 112] Administration of gadolinium-DTPA together with T1-weighted sequences in addition to bright blood time of flight sequence can be used to create maximal intensity projection (MIP) images for measurement of the degree of ICA stenosis [105-107] and accurately measure the thickness of plaque cap in relation to the necrotic core volume. [105-107] Carotid Plaque Morphology: Plaque Instability and Correlation with Development of Ischaemic Neurological Events http://dx.doi.org/10.5772/57254 95

**Figure 6.** Automated segmentation of bright–and black-blood, high-spatial resolution, Multi-contrast in vivo MR im‐ ages compared with histological characteristics of plaque. (*JACC Cardiovasc Imaging.* 2009; 2:883-96.)

High resolution MRI can identify and age intra-plaque haemorrhage. [113-115] Prospective serial MRI studies have demonstrated that haemorrhage in atherosclerotic plaques is associ‐ ated with sudden increases in plaque volume, necrotic core, and progression of degree of stenosis. [115]

In addition to Duplex and MRI, other modalities such as fludeoxyglucose (FDG) positron emission tomography (PET) CT scanning has been used to assess the level of metabolic activity in carotid atherosclerotic plaque. This is used in turn as a surrogate marker of plaque insta‐ bility.

PET CT scanning has shown some promise as a tool for assessment of plaque instability. [116] However it is unlikely to gain mainstream applicability due to its limited availability, expense, and the significant exposure to ionising radiation (meaning serial assessments are not possible) and availability of non-invasive accurate imaging modalities to assess plaque morphology.
